1. Field of the Invention
The present invention relates to a compound semiconductor device.
2. Description of the Related Art
GaN as a nitrogen-containing III-V compound semiconductor has a wide bandgap of 3.4 eV and is of a direct transition type. Therefore, GaN is expected as the material of short-wavelength light-emitting devices. Since, however, GaN has a wurtzite crystal structure and a high ionicity, it readily causes lattice defects. Further, it is difficult to obtain a low-resistance p-type GaN crystal. Especially when a p-type layer is formed by using, e.g., Mg as an acceptor impurity, hydrogen diffuses into an epitaxial layer to extremely decrease the degree of activation of the acceptor, and this makes it difficult to obtain a low resistance.
That is, it has been attempted to fabricate a double-hetero laser structure using GaN as a light-emitting layer and GaAlN as a cladding layer. In this structure, the thickness of the cladding layer required to confine light in the light-emitting layer depends upon the wavelength of the light. Since the emission wavelength of GaN is short, it is assumed that the thickness of the cladding layer can be small. For this reason, devices are normally fabricated by using thin cladding layers with a thickness of about 0.2 .mu.m.
The cladding layer, however, also plays a role of confining carriers in an active layer. According to the studies made by the present inventors, it is found that in a hetero junction constituted by nitrides such as GaAlN and GaN, the barrier height in the hetero interface is low, so the thickness of the cladding layer that is conventionally used is unsatisfactory to efficiently confine electrons and holes in the light-emitting layer. However, there is no substrate which lattice-matches with GaN. Therefore, if thick films are grown, strains caused by differences between the lattice constants and between the thermal expansion coefficients of the two substances are accumulated. The resulting increase in lattice defects makes the growth of thick films difficult.
To be more concrete, GaN is grown mostly on a sapphire substrate with a large lattice mismatch of about 15% for convenience. However, sapphire and GaN have different crystal types and a large difference in thermal expansion coefficient. For this reason, strains in the interface caused by the lattice mismatching between the substrate and GaN induce lattice defects. To reduce the influence of the lattice mismatching, therefore, various methods have been proposed.
For example, in situations where a vapor phase epitaxial (VPE) process was used as a crystal growth process, it was attempted to reduce strains in the interface with a substrate by growing a thick film about 100 .mu.m in thickness. However., no high-quality crystal could be grown because, e.g., cracks were produced. It was also attempted to form an amorphous layer on a substrate through low-temperature growth by using a metal organic chemical vapor deposition (MOCVD) process. However, the X-ray diffraction width of GaN grown by this process was very large, indicating the presence of defects at a high density. Although thick film formation was also attempted in the MOCVD growth process, the result was an increase, rather than a decrease, in defects. That is, it was impossible to grow thick films with a thickness of 3 .mu.m or more.
On the other hand, with regard to resistance of the p-type crystal, recently, it is reported that the resistance of GaN can be greatly decreased by irradiating it with an electron beam or heating it in an inert atmosphere. It is, however, difficult to obtain devices having good characteristics by these methods. That is, in the method of radiating an electron beam, electrons to be radiated must have a high energy in order to penetrate to a sufficient depth, and this readily induces crystal defects. In the case of the heat treatment, on the other hand, heating at 800.degree. C. or higher is required to sufficiently decrease the resistance. At this temperature, however, vacancies are created in an epitaxial layer by removal of N atoms, leading to lattice defects.
Even if a low resistance p-type layer is obtained, a contact resistance with an electrode and a series resistance of the device are not improved. In order to improve the device performance, it is also necessary to reduce these resistances.
Though, thus, GaN is expected as material for a light-emitting device, a formation of a thick GaN series compound layer brings about lattice defects at a high density. This lattice defects limits the thickness of a cladding layer. In addition, it is difficult to form a low-resistance p-type layer. Thus, a high-quality light-emitting device is not realized yet.